As a photocatalyst material used in the application for stain-proofing and deodorization, titanium oxide is known. The photocatalyst material is used in various fields of exterior and interior building materials, home appliances such as lighting devices, refrigerators, air-conditioners, and toilets. However, titanium oxide cannot exhibit sufficient photocatalytic performance in indoor environments having only a small amount of ultraviolet rays because it is excited by an ultraviolet region. Therefore, research and development have been in progress for a visible light responsive photocatalyst exhibiting photocatalytic performance even by visible light.
Regarding also the titanium oxide used as an ultraviolet light response-type photocatalyst, a method of doping nitrogen or sulfur in the titanium oxide, or making the titanium oxide support a metal or the like has been studied to improve performance as a visible light responsive photocatalyst. These photocatalysts cannot exhibit sufficient performance in an actual residential space in which application of photocatalyst is expected, except in the vicinity of or right below an illumination light source, since their photocatalytic activities are, under an amount of light with an illuminance of normal interior illumination (about several lx to 3000 lx), in proportion to the amount of light.
Photocatalysis is considered as an action in which light is absorbed to excite a pair of electrons and positive holes with respect to one photon, the excited electrons and positive holes activate a hydroxyl group and oxygen on a surface through oxidation-reduction, and reactive oxygen species generated by the activation perform oxidative decomposition on organic gas and the like. Therefore, the photocatalysis of the photocatalyst is lowered in an area having a small amount of light (low illuminance area). Further, an amount of gas decomposition realized by a photocatalyst is generally in proportion to the amount of light (number of photons). When an amount of irradiated light is large with respect to an organic matter on a surface, saturation occurs and an amount of photocatalysis is not in proportion to the amount of light, but, in a known photocatalyst, an amount of photocatalysts in an area having a small amount of light (low illuminance area) has a linear relation to the amount of light.
Generally, an illuminance for evaluating performance of the visible light responsive photocatalyst is equal to or more than 6000 lx. Although there is titanium oxide which is activated by visible light under such a high illuminance, since the photocatalytic activity is decreased rapidly in accordance with the decrease in the illuminance, the titanium oxide cannot exhibit sufficient performance as the visible light responsive photocatalyst under a practical environment. An application of catalyst film having a wide area is effective for indoor deodorization and removal of toxic gas such as formaldehyde, but, since an illuminance on a ceiling, a wall and a floor with wide area is low, practicality is low unless a material which exerts an effect under a low illuminance is employed.
As the visible light responsive photocatalyst, tungsten oxide is known. Patent document 1 describes a photocatalyst material made of tungsten oxide sputter-deposited on a base material, and tungsten oxide having a triclinic crystal structure is mainly used. Since the sputter deposition exposes the base material to high temperature, heat resistance temperature of some base material does not allow the application of the sputter deposition. Since a process control or the like of the sputter deposition is complicated, and it not only costs high depending on the shape and size of the base material but also has a difficulty in the deposition on a wide range such as on building materials. Moreover, though excellent in hydrophilic property, a visible light responsive photocatalyst layer made of sputter-deposited tungsten oxide has a problem that its performance of decomposing toxic gas such as acetaldehyde is not high enough. Since no data on the hydrophilic property under the irradiation of visible light is shown, it is estimated that sufficient photocatalytic performance is not obtained under visible light.
The use of a tungsten oxide powder as a photocatalyst has been also studied. If in a powder state, tungsten oxide can be mixed with a binder such as resin to be applied on a base material, which eliminates the need to expose the base material to high temperature and makes it possible to form a coating film even on a wide range such as on building materials. As a method of manufacturing the tungsten oxide powder, there has been known a method of obtaining a tungsten trioxide powder by heating ammonium paratungstate (APT) in the air (refer to Patent Document 2). The method of heating APT in the air provides a triclinic tungsten trioxide powder whose particle size is 0.01 μm (BET specific surface area=82 m2/g).
The tungsten trioxide (WO3) powder generated by the heating of APT in the air needs to have fine particles in order to have improved photocatalytic performance. However, applying a disintegration process can make the particle size small to some degree but has a difficulty in realizing the particle size of 100 nm or less, for instance. Moreover, turning it to fine powder by the use of the disintegration process causes a change in the crystal structure of the tungsten trioxide (WO3) fine powder due to a stress by the disintegration process. Since the stress of the disintegration process causes a defect of the occurrence of the re-combination of electrons and positive holes, it is thought that photocatalytic performance is deteriorated. Meanwhile, the manufacturing method described in Patent Document 2 has a problem of low manufacturing efficiency of the tungsten trioxide powder since it requires 20 hour or more kneading in order to stabilize the BET specific surface area.
As a method of efficiently obtaining a fine powder, Patent Document 3, for instance, describes a thermal plasma process. A fine powder whose particle size is 1 to 200 nm is obtained by the application of the thermal plasma process. The thermal plasma process can efficiently provide a fine powder, but even if the tungsten oxide fine powder produced by the use of the method described in Patent Document 3 is used as a photocatalyst as it is, it is not always possible to obtain a sufficient photocatalytic property. It is thought that this is because the tungsten oxide fine powder produced by the thermal plasma method does not sometimes have an optimum optical property or crystal structure.
Tungsten oxide comes in various kinds such as WO3 (tungsten trioxide), WO2 (tungsten dioxide), WO, W2O3, W4O5, and W4O11. Among them, tungsten trioxide (WO3) is mainly used as a photocatalyst material because of its excellent photocatalytic performance and its stability in a room-temperature atmosphere. However, tungsten trioxide (WO3) has a disadvantage that its photocatalytic performance is not stable because of its complicated crystal structure and its changeability by a small stress. Moreover, even if having a stable crystal structure, tungsten trioxide (WO3) cannot exhibit sufficient photocatalytic performance if its surface area is small.
Incidentally, the indoor is an environment in which an amount of ultraviolet ray is small. Further, an illuminance in the indoor is, at most, 3000 lx or less, and is several 100 lx or less except on a desk and in a workplace. For instance, a standard of the illuminance in the indoor is provided by “Recommended levels of illumination” of JIS-Z-9110 (1979) depending on each place and contents of work. According to the “Recommended levels of illumination”, a local illumination in a store, a department store and the like, and a place such as a factory in which a very delicate work is performed are defined to have a somewhat high illuminance of 1500 to 3000 lx.
However, an illuminance in a normal office, a common manufacturing process in a factory, and a place in a home where a delicate work is performed is equal to or less than 1500 lx, and further, an illuminance in a living room in a home where people enjoy gathering and a place of dining table in a dining room is low to be 500 to 150 lx. Regarding a ceiling, a wall, a floor, a furniture, a home electric appliance and the like to which the application of visible light responsive photocatalyst is expected, an illuminance of places where they are disposed is around 50 lx, which is significantly low. In particular, an illumination in a hallway and awash room is low, so that an illuminance in the vicinity of walls of such places is less than 50 lx. There is no conventional photocatalyst that exhibits practical photocatalytic performance under visible light with such a low illuminance.    [Reference 1] JP-A 2001-152130 (KOKAI)    [Reference 2] JP-A 2002-293544 (KOKAI)    [Reference 3] JP-A 2006-102737 (KOKAI)